Storing Energy in Chemical Form, the Next Step for Solar?

Capturing the sun's energy and transferring it into electricity is the key to diminishing the global demand for fossil fuels, and new research from scientists at MIT could help quicken the process further.

Traditionally the sun's energy is captured using two distinct methods; by photovoltaics, which turn the sunlight into electricity, or solar-thermal systems, which concentrate the sun's heat and use it to boil water to turn a turbine, or use the heat directly for hot water or home heating.

However scientists at MIT have been seeking alternative methods developed decades ago; namely a thermo-chemical approach which capture the configuration of certain molecules and then release the energy on demand to produce usable heat.

According to the MIT website, researchers explored thermo-chemical fuel techniques in the 1970 but were unable to find a chemical that "could reliably and reversibly switch between two states, absorbing sunlight to go into one state and then releasing heat when it reverted to the first state." In 1996 scientists discovered ruthenium, a chemical that was able to perform the necessary chemical juggling act but was too rare and expensive to be considered a viable commercial option. Ruthenium also posed scientists a challenge: they couldn't understand how it performed the energy juggling act.

However there may now be a solution as scientists at MIT have isolated the molecule fulvalene diruthenium within ruthenium, which they believe is responsible for the process. In theory, now scientists believe that now they have identified the molecule responsible, they can locate it in other elements that are more abundant and financially viable.

"Essentially, the molecule undergoes a structural transformation when it absorbs sunlight, putting the molecule into a higher-energy state where it can remain stable indefinitely. Then, triggered by a small addition of heat or a catalyst, it snaps back to its original shape, releasing heat in the process."

Compared to other approaches to solar energy "it takes many of the advantages of solar-thermal energy, but stores the heat in the form of a fuel. It's reversible, and it's stable over a long term. You can use it where you want, on demand. You could put the fuel in the sun, charge it up, then use the heat, and place the same fuel back in the sun to recharge," said Jeffrey Grossman, the Carl Richard Soderberg Associate Professor of Power Engineering in the Department of Materials Science and Engineering.

"It's my firm belief that as we understand what makes this material tick, we'll find that there will be other materials" that will work the same way.

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